[SPARK-2514] [mllib] Random RDD generator

Utilities for generating random RDDs.

RandomRDD and RandomVectorRDD are created instead of using `sc.parallelize(range:Range)` because `Range` objects in Scala can only have `size <= Int.MaxValue`. 

The object `RandomRDDGenerators` can be transformed into a generator class to reduce the number of auxiliary methods for optional arguments. 
Which do you prefer, `rng` or `generator`? `rng` feels like an instance of `RandomNumberGenerator`. As we discussed, `DistributionGenerator` is different from `RandomNumberGenerator`.